fix tests under pytest 4 Pytest 4 removed support for calling fixtures directly: <https://docs.pytest.org/en/latest/deprecations.html#calling-fixtures-directly> . This leads to an error when trying to run the tests: ``` ==================================== ERRORS ==================================== _________________ ERROR collecting tests/test_bounding_box.py __________________ Fixture "red" called directly. Fixtures are not meant to be called directly, but are created automatically when test functions request them as parameters. See https://docs.pytest.org/en/latest/fixture.html for more information about fixtures, and https://docs.pytest.org/en/latest/deprecations.html#calling-fixtures-directly about how to update your code. ```
